High power laser diodes are used as a light source in laser thermal printing systems, for color proofing systems, dry image setters, and other high quality, halftone printing systems for graphic arts applications.
In this type of laser thermal printing system, a photosensitive media is placed on a drum and is written on by a printhead which consists of multiple high power laser diodes and an imaging lens. As the image is written pixel by pixel, small areas of the photosensitive material are heated causing transfer of the material either from a donor to a receiver or from a donor to the air. In the case of plate writing, a plate is directly imaged creating hydrophobic or hydrophilic spots, after additional chemical processing and post heat treatment. The amount of dye transferred or ablated is controlled by the amount of laser energy delivered.
Laser optical power must be controlled to better than 0.5% accuracy in order to obtain a reasonable image quality. Optical power is affected by many parameters, such as laser diode driving current, operating temperature, and imaging lens system. The electronic control for the laser diode is sophisticated and complex.
In order to increase productivity, an array of laser diodes is often used in printheads. If very high laser power is required, for example, to write on slow laser thermal media, the laser driving current at nominal operating condition may be as much as 1.5 Amps per channel. A high productivity printing system with many writing channels would therefore require significant power, and generate significant heat, which would affect optical power. One attempt to deal with temperature change in a laser diode is shown in U.S. Pat. No. 5,479,425, which describes a current shunt around the laser diode. This reference, however, does not vary the temperature, only the current to the laser diode.
To maintain high productivity, driving current rise-fall time (current change from 10% to 90% of max value) should be less than 50 nanoseconds. It is difficult to achieve a rise-fall time less than 100 nanoseconds due to high stray capacitance of the driving circuit. Thus, it is seen that as the number of writing channels increases, with relatively high power requirement and fast response time, the laser diode controller electronics becomes complicated and expensive.